Integrated circuits are typically printed on silicon wafers using microlithography machines. There are two types of these machines: stepper machines and scanner machines. The light source for most of these machines currently being sold are gas discharge lasers. Currently the most used gas discharge lasers are the krypton-fluorine (KrF) excimer lasers which are pulse lasers operable at repetition rates up to about 1000 Hz or up to about 2000 Hz. The typical pulse energy is in the range of about 5 mJ to about 12 mJ. Most of these lasers now operating are the 1000 Hz models although the 2000 Hz models have now been available for about 18 months. Many of the details of a 1000 Hz KrF laser are described in U.S. Pat. No. 5,991,324. A 2000 Hz F2 laser is described in U.S. Pat. No. 6,018,537; and a 2000 Hz KrF laser is described in U.S. patent application Ser. No. 09/157,067. A technique for pulse energy control of these lasers is described in U.S. Pat. No. 6,005,879. These patents and this patent application are incorporated herein by reference. As more completely described in the above referenced documents, lasing occurs in a resonance cavity typically created between a line narrowing module and an output coupler which is typically a partially reflecting mirror. The line narrowing module typically comprises a prism beam expander and a grating for narrowing the bandwidth of the beam. In most lithography lasers, a tuning mirror is used to select the center wavelength retro reflected from the grating which is done by the angle at which the laser beam illuminates the grating. The gain medium is an electric discharge region between two elongated electrodes. Laser gas for the KrF laser is a mixture of about 0.1% fluorine, 1% krypton and the rest a buffer gas neon.
The energy of each pulse is typically controlled in an automatic feedback arrangement by adjusting the charging voltage of a pulse power system which provides a pulse discharge approximately proportional to the charging voltage. Between each discharge, the laser gas in the discharge region must be replaced. This is accomplished with a tangential blower which at 3500 rpm creates a steady gas flow between the electrodes of about 20 to 30 meters per second for 2000 Hz operation. This means that the laser gas flows about 1.25 cm between discharge at a pulse repetition rate of 2000 Hz. Since the discharge region is only about 0.5 cm wide, the products of one discharge are sufficiently moved out of the region before the subsequent discharge.
The temperature of the laser gas is controlled by a water cooled, finned heat exchanger to temperatures in the range of about 30xc2x0 C. to 60xc2x0 C. At continuous operation, for example at 1000 Hz, the temperature can be controlled without much variation, with time, although there is a temperature drop across the heat exchanger of a few degrees centigrade and a corresponding average temperature increase across the electrodes. Each pulse heats the discharge region, and this hot spot spreads out as the gas circulates around the chamber. At a blower speed of 3500 rpm, it takes about 10 to 15 milliseconds for the heated gas from a given discharge to return to the discharge region. When the laser is operating in a continuous mode, equilibrium conditions are quickly developed in the flow region around the laser chamber; however, continuous mode operation is not normal for lithography lasers. Lithography lasers are normally operated in a so-called xe2x80x9cburstxe2x80x9d mode. A typical burst mode would be xe2x80x9conxe2x80x9d for 0.15 seconds at a repetition rate of 2000 Hz (for 300 pulses), then xe2x80x9coffxe2x80x9d for 0.3 seconds while the lithography machine moves to a new die region of the wafer, then xe2x80x9conxe2x80x9d for another 0.15 second, and xe2x80x9coffxe2x80x9d again for 0.3 seconds. This operation continues until all of the die regions of the wafer (for example 120) are treated. Then the wafer is replaced with another wafer which may take a few seconds such a six seconds. Thus, one wafer per minute would be treated at this rate. This burst mode operation results in significant temperature swings in the laser gas which can directly and indirectly affect the quality of the laser beam.
Beam quality is extremely important for the lithography machines which are currently printing circuits with line widths in the range 0.25 micron. (A human hair is about 50 microns thick.) Therefore, lithography lasers are typically equipped with metrology equipment which measures for each laser pulse:
centerline wavelength
bandwidth
pulse energy
The laser also uses these values to report quality variation. Typical beam quality parameters are:
(1) Energy sigma ("sgr"E) defined as:       σ    E    =                                          ∑                          i              =              m                                      m              +              k                                ⁢                                    (                                                E                  i                                -                                  E                  T                                            )                        2                          k              /          E      T      
where m is the first pulse of a k pulse rolling window (k being the number of pulses in the window) and ET is a target pulse energy such as 10 mJ.
(2) Energy Variation from Target (EV) also called xe2x80x9cenergy stabilityxe2x80x9d defined as: EV=maximum value of Eixe2x88x92ET in a k-pulse window.
(3) Dose Variation (DV) (also called xe2x80x9cdose stabilityxe2x80x9d) defined as:       D    V    =            (                                                  ∑                              i                =                m                                            m                +                k                                      ⁢                          E              i                                k                -                  D          T                    )        /          D      T      
xe2x80x83where DT is a target dose for a k size window.
(4) Wavelength Sigma ("sgr"xcex) defined as:       σ    λ    =                              ∑                      i            =            m                                m            +            k                          ⁢                              (                                          λ                i                            -                              λ                T                                      )                    2                    k      
(5) Wavelength Variation (xcexV) also called xe2x80x9cwavelength stabilityxe2x80x9d defined as:       λ    V    =                              ∑                      i            =            m                                m            +            k                          ⁢                  λ          i                    -              λ        T              k  
(6) Bandwidth (xcex94xcex) defined as pulse spectral width at one half maximum intensity (FWHM).
By tradition, the units of "sgr"E and DV are expressed in percent. EV units are millijoules, mJ. The units of "sgr"xcex, xcex94xcex and xcexV are picometers, pm.
These values are stored temporarily in a memory buffer of the laser controller and can be read out to an external information processor or storage device or can be read by the stepper/scanner as desired.
Typical specifications for a KrF excimer laser might be:
These specifications are examples of the type of quality standards which are applied to determine if a laser""s performance passes an acceptance test prior to shipment from the laser fabrication plant. These values are sometimes reported as maximum values during a specified period of time. The sigma values are typically reported as xe2x80x9c3 sigmaxe2x80x9d values.
The present invention provides a lithograph quality optimization process for controlling laser beam parameters when changing operating modes. The laser is programmed to automatically conduct an optimization procedure preferably in less than one minute to adjust laser operating parameters such as blower speed, total gas pressure and F2 partial pressure in order to optimize beam quality parameters.